What defines a hydrocarbon?

A hydrocarbon is defined as a molecule that consists entirely of carbon and hydrogen atoms. This definition is critical in organic chemistry, as hydrocarbons serve as the foundational building blocks for a wide variety of organic compounds, including fuels, plastics, and pharmaceuticals. The structure of hydrocarbons can vary greatly, leading to different types, such as alkanes, alkenes, and alkynes, which differ in the types of bonds between the carbon atoms and the number of hydrogen atoms attached.

The other choices illustrate different combinations of elements that do not fit the definition of hydrocarbons. For instance, a molecule containing only carbon and oxygen would be characteristic of compounds like carbon dioxide or carbon monoxide, which are not hydrocarbons. A molecule that includes only oxygen and hydrogen describes water or hydrogen peroxide, both of which contain no carbon. Lastly, a molecule that includes metals does not pertain to hydrocarbons, as hydrocarbons are specifically characterized by the absence of metals and the presence of only carbon and hydrogen.